(a) Field of the invention
This invention relates to a small signal amplifying circuit and, more particularly, a small signal amplifying circuit suitable for use in a system in which metal contact portions such as connectors are used between a signal source and an amplifier.
(b) Description of the prior art
Various connectors using metal contacts are provided in a signal path in a disc player-amplifier system for enabling a listener to change freely a cartridge, a head shell, a tone arm or even an entire player. Since the output signal of a phonocartridge, particularly that of a moving coil type phonocartridge, is a very small signal, this output signal is liable to be affected by variation in electric characteristics of the metal contact portions and, as a result, the signal is transmitted non-linearly accompanied by distortion. If the level of the output signal is relatively large, an oxide-coated layer produced on the surface of the metal contact portion, for instance, will not affect the signal so much owing to dielectric breakdown of the insulation posed by such oxide-coated layer. Such dielectric breakdown, however, cannot be expected in an output level of a moving coil type cartridge, i.e., in the order of several hundred .mu.V.
A proposal for overcoming such inconvenience has been made in Japanese Patent Preliminary Publication No. Sho 54-120516. This proposal is based on the fact that a non-linear portion in voltage-current characteristic in a metal contact portion exists in the vicinity of 0 voltage in a case where a signal is transmitted through a signal path having the metal contact portion and attempts to transmit the signal by superposing it on a direct current of such a level that the signal will not be affected by the non-linear portion produced by the metal contact portion. Since, however, negative feedback cannot be applied in the circuit which is specifically shown as the embodiment of the disclosed invention in the publication, this circuit is disadvantageous in distortion factor, signal-to-noise ratio, irregularity in gain and stability of characteristics. If mutual conductance gm of a transistor as an amplifying element is increased for improving the signal-to-noise ratio, it will restrict a maximum allowable input level. Further, the irregularity in gain and the stability of characteristics are considered to depend upon properties of active elements so that it will be difficult to ensure a uniform gain and stable characteristics of the circuit in a large scale production of such active elements.